The new levels are unlocked via in game achievements. To unlock the bonus levels for an episode, you need to finish all the levels and or collect all the fruit.
Further still, a statement in the Google Play Store listing from Rovio says that "this is not the final update for Angry Birds Rio, so stay tuned." Like it or not, Rovio are one of the best out there at giving us new content. Download links can be found after the break.